Helicases in R-loop Formation and Resolution

With the development and wide usage of CRISPR technol-ogy, the presence of R-loop structures, which consist of an RNA-DNA hybrid and a displaced single-strand (ss) DNA, has become well accepted. R-loop structures have been implicated in a variety of circumstances and play critical roles in the metabolism of nucleic acid and relevant biological processes, including transcription, DNA repair, and telomere mainte-nance. Helicases are enzymes that use an ATP-driven motor force to unwind double-strand (ds) DNA, dsRNA, or RNA- DNA hybrids. Additionally, certain helicases have strand -annealing activity. Thus, helicases possess unique positions for R-loop biogenesis: they utilize their strand-annealing ac-tivity to promote the hybridization of RNA to DNA, leading to the formation of R-loops; conversely, they utilize their un-winding activity to separate RNA-DNA hybrids and resolve R-loops. Indeed, numerous helicases such as senataxin (SETX), Aquarius (AQR), WRN, BLM, RTEL1, PIF1, FANCM, ATRX (alpha-thalassemia/mental retardation, X-linked), CasDinG, and several DEAD/H-box proteins are reported to resolve R-loops; while other helicases, such as Cas3 and UPF1, are reported to stimulate R-loop formation. Moreover, helicases like DDX1, DDX17, and DHX9 have been identified in both R-loop formation and resolution. In this review, we will sum-marize the latest understandings regarding the roles of heli-cases in R-loop metabolism. Additionally, we will highlight challenges associated with drug discovery in the context of targeting these R-loop helicases.